It is well known that steel is produced today through three fundamental procedures.
The first procedure is called integrated cycle or primary steelmaking. Here the ferrous mineral (usually an iron oxide, in particular Fe2O3), by means of the blast furnace where it comes into contact with coke and other additives (mainly CaCO3) is transformed into pig iron. Downstream of the blast furnace the pig iron is worked to reduce the percentage of carbon and thus convert it into steel.
A second cycle in steelmaking, known as secondary steelmaking, sets out from scrap to regenerate it and obtain new steel. The heart of this steelmaking is the electric arc furnace (EAF) where the scrap is melted. Downstream of the electric furnace subsequent work processes result in semi-manufactured product.
Then there is an intermediate procedure which consists in direct reduction of the iron oxides to metallic iron, without fusion of the latter. In this way so called sponge iron is obtained, also known as Direct Reduced Iron (DRI). This sponge may then be worked to remove impurities (e.g. inclusion of inert elements) and form hot briquetted iron (HBI) with which to feed the secondary steelmaking cycle.
Currently global steel production is equally divided between primary and secondary steelmaking while the amount of steel produced using raw material from direct reduction is decidedly less.
Each of the procedures described above has its pros and cons. For example primary steelmaking calls for considerable initial investments due to the costs of a blast furnace and the necessary infrastructures for raw materials supply. On the other hand a blast furnace has relatively low management costs and great production capacity, in the order of several thousand tonnes of pig iron a day.
Moreover, steel obtained from primary steelmaking is usually of excellent quality and is preferred for many uses. For example in a sector of strategic importance like the automobile industry, blast furnace steel is clearly preferred. In fact in car manufacture the working of relatively thin sheet metal and the conformation of fairly narrow curves must give aesthetically pleasing results. These features are guaranteed with the use of blast furnace steel.
Secondary steelmaking is in fact based on plant smaller than the blast furnace. So the individual plant requires less initial investment but has lower production capacity, usually in the order of some hundreds or a few thousand of tonnes of steel a day.
An ordinary EAF consists essentially of a lower chamber to collect the molten steel, an upper chamber consisting of cooled panels which generates the material to be loaded, a retractable roof through which three electrodes enter the furnace and an exhaust suction system.
The electric arc furnace has considerable advantages over the blast furnace. Firstly, it is fed chiefly by scrap, thus playing a fundamental role in recycling raw material, with clear environmental advantages. Moreover the EAF has evolved continually over the last forty years into an extremely efficient system. In particular there have been progressive improvements in energy efficiency and continual reductions in management costs and environmental impact.
These indubitable advantages of secondary steelmaking however are countered by certain negative aspects, above all due to the fact that the ordinary melting cycle that takes place in an electric arc furnace involves power off times in which the furnace is not functioning.
Currently the average cycle of an EAF is around 40-60 minutes from tap to tap. During a single cycle the furnace melts on average the contents of two or three feed baskets.
Feeding in the contents of each basket requires that the electrodes be switched off and removed and the retractable roof of the furnace raised. These operations involve an overall power off time of about 10-20% of cycle duration.
Furthermore the volume of each basket is practically equal to that of the furnace. A certain mass in scrap in fact occupies a far greater volume (roughly ten times) that it does in the state of molten steel.
This is why electric furnaces of the well known kind comprise, above the bowl for collecting the molten steel, an upper chamber for the great volume of scrap inserted at each loading. While the bowl has a refractory lining the upper chamber is usually in metal panels, suitably cooled. It should be pointed out that about 10-15% of the total energy introduced into the furnace is removed and dispersed in the form of heat for cooling the cooled parts.
Lastly, at the moment of tapping, electric furnaces of the well known kind are tipped up to pour the melt out through a special aperture. In this operation the roof is kept closed to avoid excessive heat dispersion. During tapping the roof remains closed and the electric power supply is switched off.
Given the foregoing, in furnaces of the well known kind the position of the exhaust system intake on the roof is limited to one position only. It must be placed in such a way as to coincide with the axis of the sleeve of the exhaust suction tubing downstream of the furnace. This configuration prevents the conduit from maintaining its position during tapping.
It should be pointed out that the exhaust system carries off about 2% of the mass loaded into the furnace from the baskets. Dust, the lightest shavings and the smallest fragments of metal may easily be sucked in with the exhaust gases. This means a clear loss of steel produced and extremely strenuous working conditions for the suction plant filters.
There are well known solutions for loading the furnace, not with baskets but with continuous feed systems such as mechanical loaders, conveyor belts and similar. The furnace loading points are in the upper chamber, therefore it is loaded from the side of the furnace or from above through the roof.
Though with these solutions there is no longer a need to open the roof for loading, mechanical loaders or conveyor belts—as well as the electrodes in certain cases—must be removed at the moment of tapping to avoid interference.
Moreover if the furnace feed hatch is at the side, over and above the mechanical difficulties mentioned previously there is a clear thermal unbalance due to the lateral position (asymmetrical) of the material to be melted. And since the loading hatch is close to the level of the melt, it is subject not only to a considerable thermal load but also to the risk of being filled with slag during the uncontrolled reactions that take place inside the furnace. Where loading is done through the roof there are further problems such as: the loading position close to the exhaust intake; the height from which the scrap is dropped causing spurts of molten material, and the considerable plant engineering complications of a roof which, as we recall, must rotate or roto-translate with the furnace during tapping and deslagging operations. Furthermore the propinquity of the loading hatch to the exhaust intake increases the percentage of fine material aspirated.